The present invention relates to a centrifugal extractor capable of liquid-liquid extraction by bringing an organic phase into contact with an aqueous phase and utilizing centrifugal force. More particularly, the present invention relates to an improved centrifugal extractor capable of obtaining high extraction efficiency even when a flow rate ratio of both phases is great.
In a reprocessing of a spent nuclear fuel and in a nuclide separation recovery process, the centrifugal extractor of the present invention can advantageously be used particularly in a system having a large flow rate ratio such as for solvent washing and dodecane washing. However, the extractor of the present invention is not limited to this application and can widely be used for liquid-liquid extraction by bringing an organic phase and an aqueous phase into mutual contact.
A centrifugal extractor forcibly separates a mixed solution of an organic phase and an aqueous phase by centrifugal force, and its typical structure is shown in FIG. 9. The centrifugal extractor shown in the drawing fundamentally comprises a cylindrical casing 1 and a cylindrical rotor 3 which is rotated at a high speed by a rotary shaft 2 inside the casing 1. An organic phase O and an aqueous phase A are supplied to a mixing portion 6 at the bottom of the casing 1 from an organic phase inlet 4 and an aqueous phase inlet 5, respectively, and are mixed between the casing 1 and the rotating rotor 3. The mixed solution is introduced into a phase separation portion 8 from a supply port 7 at a lower part of the rotor. The aqueous phase A having a greater specific gravity is separated outward while the organic phase O having a smaller specific gravity is separated inward, and they rise upwards along the inner peripheral surface of the rotor 1. The aqueous phase A having a greater specific gravity and existing on the outer side of an interface K overflows from a weir 9 for discharging the aqueous phase and is discharged outside the casing from an aqueous phase draw port 10 and an aqueous phase outlet 11. The organic phase O having a smaller specific gravity and existing on the inner side of the interface K overflows from a weir 12 for discharging the organic phase and is discharged from an organic phnase draw port 13 and an organic phase outlet 14 outside the casing 1. The organic phase and the aqueous phase thus discharged are sent to a centrifugal extractor of the next stage, if required, and subjected to multi-stage extraction.
FIG. 10 is a flow diagram of a multi-stage counter flow system which disposes conventional centrifugal extractors in multiple stages and brings the organic phase and the aqueous phase into counter flow contact with each other. Each centrifugal extractor comprises a mixing portion M and a phase separation portion S, and a plurality of such extractors are disposed in multiple stages such as an i-1 stage, an i stage, an i+1 stage, an i+2 stage, and so forth. The organic phase is introduced into the extractor of the i-1 stage and is finally discharged from the extractor of the i+2 stage. On the other hand, the aqueous phase is introduced into the i+2 stage extractor and is finally discharged from the i-1 stage extractor. The flow rate ratio in the mixing portion M and phase separation portion S of the extractor of each stage is primarily determined by the flow rate F.sub.o of the organic phase and the flow rate F.sub.a of the aqueous phase and can be expressed by F.sub.a / F.sub.o. In the case of dodecane washing where F.sub.o is extremely smaller than F.sub.a, for example, F.sub.a (flow rate of the aqueous phase) / F.sub.o (flow rate of dodecane) becomes about 100. For this reason, sufficient contact and mixing between both phases cannot be achieved inside the mixing portion M of the extractor (mixing portion 6 shown in FIG. 9) and a limit is imposed on extraction efficiency.
When the flow rate ratio becomes great (or in other words, when the flow rate of the aqueous phase becomes greater than that of the organic phase), the interface K inside the rotor 3 (see FIG. 9) moves towards the center of the rotor, and mixture of the aqueous phase into the discharged organic phase (entrainment) increases. In the reverse case, the interface K moves towards the inner peripheral surface side of the rotor and mixture of the organic phase into the discharged aqueous phase increases.
A structure capable of adjusting the height of the weir in order to prevent destabilization of the interface K inside the rotor due to fluctuation of the flow rate ratio has been proposed (Japanese Patent Publication No. 55985/1988). In the case of a fixed weir, there have also been proposed a structure which enlarges the size of a severer condition side, that is, the size of the extractor, so as to provide a greater margin, and a structure which disposes a weir chamber and blows compressed air from outside into this weir chamber through a rotary shaft in order to regulate the pressure and thus to control the interface K to be disposed at an optimum position.
However, in the case of the structure wherein compressed air is supplied, seals between the rotating rotary shaft and the compressed air supply portion and pipings for supplying compressed air to each stage are necessary, so that the structure becomes complicated and moreover, cannot be made compact. In the case of the structure capable of adjusting the height of the weir, the structure of the weir becomes complicated.